To produce pipes, plastics material is melted in an extruder and pressed by a corresponding mould. In a subsequent calibration station, the outer diameter produced in this way is frozen and in water spray baths or full baths the heat is removed from the plastics material via the outer surface. In this case, all the heat must be conducted through the plastics material wall towards the outside, where it is then dissipated by cooling water. The cooling length is disproportionally increased as the pipe wall thickness increases, since the plastics material is a poor heat conductor. In addition, the temperature inside the pipe remains high for a long time and the plastics material has sufficient time to flow downwards owing to gravity. As a result, the pipe wall is always thinner in the upper region than in the lower region.
This process thus has two drawbacks as the pipe wall thickness increases. The cooling length is disproportionately increased and the plastics material flows down the inner wall, resulting in a non-uniform wall thickness distribution. To reduce these drawbacks, a plurality of systems for cooling pipe interiors are described in, for example EP 795 389. However, these systems are very limited in terms of their operation. Since the pipe is cooled both outside and inside, both the outer contour and the inner contour are frozen. If the melt in the centre of the pipe wall then solidifies owing to outward and inward heat dissipation, the severe volume reduction at the transition from the molten state to the solid state results in cavities. This risk increases with increasing pipe wall thicknesses.
JP 56-005 750 A describes a device for extruding plastics material profiles, to which device plasticised polymer melt can be fed and which comprises a plurality of annular conduits which are united to form a common melt conduit. Cooling conduits are arranged around these annular conduits.
DE 10 2005 031 747 A1 describes a method for the internal cooling of hollow plastics material profiles and an extruder for producing hollow plastics material profiles. In this case the internal cooling is achieved by conveying a cooling gas into the interior of the hollow profile, the cooling gas being produced in a Ranque-Hilsch vortex tube.
DE 10 2007 050 291 describes a mould for additional cooling of the melt, in which mould the melt strand is divided into a plurality of substrands.
In terms of processing, for example owing to the division into a plurality of flow conduits (which may all be cooled), the above-mentioned solutions are difficult to handle and involve very high production costs and a considerably high level of complexity when maintaining and cleaning the flow conduits. In addition, the risk of the melt being able to mix with the coolant is very high, since the two flow chambers must be operated at very high internal pressures during extrusion operation and said chambers are separated from each other only by a simple steel surface seal.